A Level Physics ELECTROMAGNETISM Magnetic fields due to currents.

MAGNETIC FIELDS DUE TO CURRENTS
A flowing electric current has a magnetic field around it, if conventional current flow ( + to -) is used then the field is clockwise round the wire. This is the same way as screws go in, or the way your fingers grip on your right hand if your thumb goes the same direction as the current. The equation for the flux density B is

B = u0 I / 2 πa
where I is the current, a is the distance from the wire and u0 is the permeability.

A coil has a similar field to a fixed bar magnet, again this can be remebered by using a right hand grip, this time fingers resprest the current and the thumb is in the direction of the field. The equation for flux density is

B = u0 N I / l
where N is the number of turns and l is the length of the solenoid. Sometimes N / l is written as n, the turns per unit length.

A plane circular coil has a similar pattern to a coil only with no length, the equation for flux density at its center is

B = u0 N I / 2 r
where r is the radius of the coil. The WJEC don`t expect you to memorize these equations.

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Rob Coles 28 May 1996 10:42:11